Electronic humidity chamber for vapor desorption to determine high capillary pressures

ABSTRACT

The present invention relates to the use of an electronically controlled humidity chamber with temperature controls: to determine, by vapor desorption, high capillary pressures of core samples; to produce core samples with a high capillary pressure for testing electrical properties; and, using a curve of high capillary pressures from vapor desorption data, to transform a curve of high capillary pressures determined from high pressure mercury injection.

RELATED APPLICATIONS

The present application is a second Divisional of U.S. patentapplication Ser. No. 10/956,809, filed Oct. 1, 2004, entitled“ELECTRONIC HUMIDITY CHAMBER FOR VAPOR DESORPTION TO DETERMINE HIGHCAPILLARY PRESSURES,” now, U.S. Pat. No. 7,171,843. The first Divisionalis U.S. patent application Ser. No. 11/645,460, filed Dec. 26, 2006,now, U.S. Pat. No. 7,406,857.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This present invention relates to the use of an electronicallycontrolled humidity chamber with temperature controls to collect vapordesorption data which is used to calculate the capillary pressure of acore sample. In another aspect of the present invention, a core samplewith a high capillary pressure is produced and at least one electricalproperty is determined. In another aspect of the present invention, thecapillary pressures determined according to the present invention areused to transform capillary pressures determined from the high pressuremercury injection method.

2. Description of the Related Art

Knowledge of capillary pressure or each specific rock/oil/watercombination present in reservoir rocks is highly important forpredicting potential hydrocarbon in place within a reservoir. Capillarypressure data is a measure of the interaction between fluids and therock pore surface. The strength of capillary interaction varies with thefluid saturations, the interfacial tension between the fluids, the porestructure, and the wettability of the pore surfaces.

Capillary pressure measurements also provide basic descriptions of thereservoir rock, fluids and rock-fluid behavior. Capillary pressure datamay be used to: estimate pore throat size distribution to classifyhydraulic rock types; determine initial water saturation conditions;estimate water saturation, permeability, porosity, and height above freewater level for reserve estimates; estimate absolute permeability;estimate seal capacity of the sealing facies; and estimate capillarypressure water saturation profiles.

Several techniques have been developed for measuring capillary pressuresof core samples, including porous plate, centrifuge and mercuryinjection. As known by one of skill in the art, the porous plate andcentrifuge methods have the advantage of being able to use reservoirfluids during the capillary pressure measurements; however, limitationson the maximum achievable capillary pressure preclude application insituations where high capillary pressure exists, such as tight gassands. The porous plate and centrifuge methods are generally limited tocapillary pressures up to about 1000 psi.

High-pressure mercury-injection can reach the necessary pressures,typically 5000 to 10,000 psi, but the use of non-reservoir fluids tocompute capillary pressures produces inaccurate results andtransformation is required to correct the capillary pressure data. Theinaccurate results are believed to be due to the lack of a true wettingphase during testing. The test is performed on dry samples using mercuryas the non-wetting phase fluid and assuming air is the wetting liquid.This requires conversion to reservoir conditions using contact angle andsurface tension inputs. Additionally, the oil and gas industry lacks aconsensus of standards for correcting system compressibility at highpressures resulting in water saturation/capillary pressure distributionmeasurement uncertainties. Finally, use of the contact angle and surfacetension scaling parameters are generally not appropriate for rocks withultra-low water saturations and high capillary pressures or rocks commonto tight gas sand reservoirs.

It is also known that the vapor desorption method can be used tocalculate capillary pressures. While the vapor desorption methodproduces accurate results at moderately high capillary pressures andmoderately low water saturations, the vapor desorption measurementprecision decreases at very high relative humidity (>95%) which limitsthe lower limit of capillary pressure to a range of approximately 1000psi. Thus, a disadvantage of the vapor desorption technique is theinability to measure capillary pressures at high water saturations.

For the vapor desorption method, it is known that the Kelvinrelationship: Pc=ln(RH/100)RT/Vm, (where: Pc is the capillary pressure,psi; RH is the relative humidity; R is the universal gas constant, 8.314J/Mol K; T is the absolute temperature, degrees Kelvin; and Vm is themolar volume of water) can be used to compute air/brine capillarypressures for core samples. (The Kelvin relationship is known to thoseof skill in the art.) This is detailed in SPE Paper No. 16286, “Use ofWater Vapor Desorption Data in the Determination of CapillaryPressures”. Experimentally, the core samples are allowed to reachequilibrium in a constant vapor pressure environment. As discussed inthe paper, a known way to establish the constant vapor pressureenvironment is to use saturated solutions of salts such as BaCl₂, KNO₃,and K₂SO₄. Using the vapor pressure data for these solutions, the Kelvincapillary pressures are calculated for a range of NaCl brinecompositions and a range of temperatures. The lowest humidity levelshown in the paper is 0.8987 produced by a saturated solution of BaCl₂at 30° C. Applicants are not aware of salt solutions that will producepractical humidity levels below this 0.8987 level. The paper discussesthe use of salt solutions to calculate capillary pressures as high as4000 psi.

Electrical properties such as formation factor and resistivity index areoften calculated at a number of varying water saturations or capillarypressures. However, because the porous plate and centrifuge methods arelimited on the maximum capillary pressure, the calculation of theseelectrical properties has been limited to high water saturations and lowcapillary pressures.

Thus, there are a number of shortcomings with the prior art, including:the inability to accurately determine high capillary pressures; theinability to produce core samples having low water saturation and highcapillary pressure for measurement of electrical properties; and theinability to obtain accurate capillary pressures using the high-pressuremercury injection method.

SUMMARY OF THE INVENTION

Accordingly, a need has arisen for a method of determining highcapillary pressure, e.g., in excess of 4,200 psi and the correspondinglow water saturations, e.g., approximately 5%. A further need exists fordetermining electrical properties of core samples at high capillarypressures. A further need exists for a method of transforming thecapillary pressures determined from high-pressure mercury injection,particularly at high capillary pressures.

In accordance with present invention, an electronically controlledhumidity chamber with temperature controls is used to control humidityin a method using vapor desorption to determine capillary pressures of acore sample or to produce high capillary pressure within a core sample.

Accordingly, an object of the present invention is to use anelectronically controlled humidity chamber with temperature controls toprovide a method of using vapor desorption to measure high capillarypressures, e.g., in excess of 4,200 psi and at low water saturations,e.g., below 5%.

A further object of the present invention is to use an electronicallycontrolled humidity chamber with temperature controls to produce coresamples having low water saturation and high capillary pressures. Theelectrical properties of these core samples may then be determined.

A still further object of present invention is to use an electronicallycontrolled humidity chamber with temperature controls to collect vapordesorption data to determine the capillary pressures of core sampleswhich can then be used to transform the capillary pressure datadetermined according to the high-pressure mercury injection method.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the invention can be obtained when thedetailed description of exemplary embodiments set forth below isconsidered in conjunction with the attached drawings in which:

FIG. 1 is a graphical presentation of capillary pressures fromExperiment 1 reported herein.

FIG. 2 is a graphical presentation of capillary pressures fromExperiment 3 reported herein.

FIG. 3 is a graphical presentation of capillary pressures fromExperiment 3 reported herein.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

The present invention relates to a method of using an electronicallycontrolled humidity chamber with temperature controls to collect vapordesorption data which is used to calculate the capillary pressure of acore sample. The electronically controlled humidity chamber has anelectronic relative humidity sensor and water is sprayed into thechamber to maintain the humidity at the desired level. The humiditychamber controls both humidity and temperature electronically. Humidityis maintained within 1 percentage unit and temperature is controlledwithin 1 degree. The humidity chamber can be set at a relative humidity(RH) from 90% (which corresponds to a capillary pressure ofapproximately 2,200 psi) to a RH of 55% (which corresponds to acapillary pressure of approximately 12,500-14,000 psi). In fact, thehumidity chamber can be controlled to a RH of approximately 25% (whichcorresponds to a capillary pressure of approximately 30,000 psi). Thetemperature range can be controlled between 25° C.-60° C.

Tight gas sands constitute a significant percentage of the U.S. naturalgas base and offer tremendous potential for future reserve growth andproduction. Tight gas sands often exhibit unique gas storage andproducing characteristics. Many tight gas sands are also characterizedby very low connate water saturations and associated high capillarypressures. Consequently, effective exploitation of these resourcesrequires accurate descriptions of key reservoir parameters, particularlycapillary pressures, to quantify the vertical water saturationdistribution and resource-in-place. As noted above, the prior art doesnot provide a method for accurately determining the high capillarypressures associated with these low water saturations.

In accordance with the present invention, a method for determining thecapillary pressure of a core sample most preferably at or above about4,200 psi (also preferably at or above 5,000 psi) (further preferably ator above about 6,000 psi) is provided, the method comprising the stepsof: initially desaturating the core sample at a capillary pressure of1000 psi, placing the core sample in an electronically controlledhumidity chamber with temperature controls, setting a humidity level inthe humidity chamber at about 80 percent or lower, periodically weighingthe core sample to determine when equilibrium water saturation isachieved at the humidity level, and calculating the capillary pressureat 2000 psi. The vapor desorption process is continued in a step-wisemanner at decreasing relative humidity levels yielding increasingcapillary pressures to an approximate maximum capillary pressure ofapproximately 14,000 psi.

As is known to one of skills in the art, electrical properties of coresamples, e.g., formation factor, resistivity index, cementation exponentand saturation exponent, are important in determining the water or oiland/or gas reserves in place. Prior to the present invention, there wasno known way to produce core samples having high capillary pressures (ator above 4200 psi) and low water saturations (below about 5%) on whichelectrical properties could be tested. Vapor desorption using saltsolutions is limited to a capillary pressure of 4000 psi. The centrifugeand porous plate methods are limited to capillary pressures of about1000 psi. While high-pressure mercury injection is capable of measuringcapillary pressures up to 60,000 psi, it is a destructive testing methodand the resulting core samples are not suitable for testing electricalproperties.

In accordance with the present intention, a method for determining anelectrical property of a core sample having a capillary pressure aboveabout 4,200 psi is provided, comprising the steps: Initially determiningelectrical properties using conventional methods to a maximum capillarypressure of 1000 psi. The desaturation process is then continued byplacing the core sample in an electronically controlled humidity chamberwith temperature controls, setting a humidity level in the humiditychamber at about 80%, periodically weighing the core sample to determinewhen equilibrium water saturation is achieved at the humidity level,calculating the capillary pressure at or above about 4,200 psi, andmeasuring an electrical property of the core sample at that saturationlevel. The desaturation process is then continued at a lower relativehumidity percentages (higher capillary pressures) in the humiditychamber and the electrical properties determined at each stabilitysaturation point.

The determination of high-pressure mercury injection capillary pressures(MICP) involves injecting or forcing mercury into an evacuated coresample in a step-wise manner from vacuum to 60000 psi air-mercury. Thevolume of mercury injected at each pressure determines the non-wetting(i.e., mercury) saturation. Then, as known by one of skill in the art,the corresponding wetting phase capillary pressure is calculated at eachmercury injection pressure.

In accordance with the present invention, a method for building atransform between wetting fluid saturations and those non-wettingsaturations determined using high pressure mercury injection isprovided, comprising the steps of: in a step wise fashion, injectingmercury into an evacuated core sample at a number of pressures,measuring the volume of mercury injected at each pressure, calculatingthe capillary pressure by high-pressure mercury injection at eachpressure, in a step wise fashion, using an electronically controlledhumidity chamber with temperature controls to collect vapor desorptiondata, using the vapor desorption data, calculating a step wise series ofcapillary pressures at or above about 4,200 psi, using the capillarypressures calculated from the vapor desorption data, transforming thecapillary pressure data determined by high pressure mercury injection.The formula for transforming the high-pressure mercury injectioncapillary pressure is:

$P_{cAW} = {P_{cAM}\left( \frac{2\;\sigma_{AW}\cos\;\theta_{AW}}{2\;\sigma_{AM}\cos\;\theta_{AM}} \right)}$(where: P_(cAW)=pseudo air-water capillary pressure, psi;P_(cAM)=air-mercury capillary pressure, psi; σ_(AW)=air-water surfacetension; σ_(AM)=air-mercury surface tension; cos θ_(AW)=air-watercontact angle; cos θ_(AM)=air-mercury contact angle) further comprisingthe steps of: graphing the capillary pressures calculated from the vapordesorption data to produce a vapor desorption curve; graphing thecapillary pressures determined from high pressure mercury to product ahigh-pressure mercury injection curve; adjusting the σ_(AM) cos θ_(AM)term until the high-pressure mercury injection curve closely matches thevapor desorption curve.

EXPERIMENTAL Experiment 1

Several core samples from the Bossier tight gas sand play in the MimmsCreek Field located in Freestone Co., Texas were analyzed and theresults are presented below.

Procedure:

-   -   1. Clean subject samples using cycles of a miscible solvent        sequence to remove all hydrocarbons and aqueous pore fluids.    -   2. Dry to stable weight and determine basic sample properties.    -   3. Vacuum saturate sample with synthetic brine. The brine        salinity is chosen so the precipitation will not occur during        the vapor desorption process.    -   4. Pressurize pore volumes or back pressure flush to ensure 100%        brine saturation.    -   5. Generate a low-pressure capillary pressure curve to a maximum        of 1,000 psi using either porous plate or centrifuge methods.    -   6. Record the final sample weights at irreducible water        saturation determined at test conditions (Swi).    -   7. Place samples into the electronically controlled humidity        chamber at the 1^(st) (highest) relative humidity set point        (approximately 90% RH)    -   8. Monitor sample weights daily until stable. Calculate average        brine saturation for each sample based on the Swi weight minus        the sample dry weight divided by the sample pore volume.        Calculate brine salinity in each sample.    -   9. The capillary pressures are calculated using a modified        Kelvin equation: Pc=ln(RH/100)RT/Vm.    -   10. Repeat steps 7 through 9 at the next lower oven RH setpoint        (typically 80% RH).    -   11. Repeat steps 7 through 9 at the next lower oven RH setpoint        (typically 70% RH).    -   12. Repeat steps 7 through 9 at the next lower oven RH setpoint        (typically 60% RH).    -   13. Report vapor desorption based brine saturations vs capillary        pressure for each sample.        Results:

TABLE 1 Capillary Pressure by Centrifuge Method Sample Sample SampleSample Sample Sample Approximate HB1-26 HB2-28 HB2-32 HB3-17 HB3-42HB4-18 Average Satn. Satn. Satn. Satn. Satn. Satn. Capillary fract fractFract Fract Fract fract Pressure 0.9110 0.9900 0.7410 0.4820 0.53700.6520 100 0.8500 0.9500 0.5710 0.3000 0.3800 0.6220 200 0.7450 0.86300.4390 0.1960 0.2690 0.5680 400 0.5260 0.7680 0.3110 0.1150 0.17000.4420 1000

TABLE 2 Capillary Pressure Using an Electronically Controlled HumidityChamber to Collect Vapor Desorption Data Sample No. HB1-26 Sample No.HB2-28 Sample No. HB2-32 Capillary Capillary Capillary Relative Satn,Pressure, Satn, Pressure, Satn. Pressure, Humidity, % Weight, g fractpsi Weight, g fract psi Weight, g fract psi 90% 44.1300 0.3582 221768.6420 0.5812 2226 61.7240 0.2130 2218 80% 43.9340 0.1499 4725 68.17900.2322 4738 61.4430 0.0837 4712 70% 43.8670 0.0786 7593 68.0280 0.11837606 61.3530 0.0423 7550 60% 43.8310 0.0404 10967 67.9450 0.0558 1091561.3040 0.0198 10775 Sample No. HB3-17 Sample No. HB3-42 Sample No.HB4-18 Average Capillary Capillary Capillary Capillary Relative Satn.Pressure, Satn. Pressure, Satn. Pressure, Pressures, Humidity, % Weight,g fract psi Weight, g fract psi Weight, g fract psi psi 90% 62.24800.0605 2191 63.2970 0.0941 2197 57.3160 0.2841 2212 2210 80% 62.09500.0209 4600 63.0770 0.0334 4625 56.8900 0.1066 4685 4681 70% 62.05400.0103 7298 63.0150 0.0163 7344 56.7650 0.0546 7500 7482 60% 62.03600.0057 10445 62.9860 0.0083 10422 56.7010 0.0279 10767 10715

FIG. 1 is a graphical presentation of the above capillary pressuresusing the average capillary pressures, wherein the capillary pressuresup to 1,000 psi (Table 1) were determined using the centrifuge methodand capillary pressures above 1,000 psi (Table 2) were determined usingthe method of the present invention. Although there is no direct overlapor connection between the vapor desorption data of the present inventionand the centrifuge data, they do appear to follow a consistent trend,thus validating the vapor desorption measurements of the presentinvention. Also, although there is no direct overlap in the data, onecan observe a very clear and obvious continuity in the compositecapillary curves. It is shown that the two methods are in continuum atthe capillary pressure range of 1000-2000 psi for the six samplestested. The merged data satisfactorily provide drainage curves that spanthe complete saturation range. This continuity provides confidence thatthe vapor desorption method provides an accurate description of thesaturation distribution in the low water saturation and high capillaryregion of the capillary pressure curve.

The experiment also shows that core samples having both low watersaturations and high capillary pressures can be produced in thelaboratory using the vapor desorption method of the present invention.

Experiment 2

The electrical properties from a core sample from the Dowdy Ranch Fieldin Freestone Co., Texas were measured for varying capillary pressures.

TABLE 3 Sample 218 Rw, ohm-m @25° C.: 0.1237 Water A/B Saturation,Capillary Resistivity Index Fraction Pressure, Leverett J Rt, R1Incremental pv psi Function ohm-m (Rt/Ro) Saturation n 0.956 100 0.1529.66 1.058 −1.25 0.914 140 0.21 32.57 1.162 −1.67 0.822 200 0.30 39.191.398 −1.71 0.545 400 0.59 73.86 2.635 −1.60 0.303 700 1.03 174.0 6.206−1.53 0.263 1000 1.48 207.0 7.384 −1.50 0.142 2105 3.11 362.5 21.14−1.56 0.082 4506 6.65 690.1 60.47 −1.64 0.046 7163 10.6 1429 161.5 −1.650.035 10268 15.2 2321 251.0 −1.65 Formation Factor FF(Ro/Rw) = 226.6 Ro,ohm-m = 28.03 Cementation Exponent, m = −2.01 Saturation Exponent, n =−1.57

In this experiment, the capillary pressures up to 1000 psi weredetermined using the centrifuge method while the capillary pressuresabove 1000 psi were determined by the method described herein using anelectronically controlled humidity chamber with temperature controls.

Experiment 3

Several core samples from North Louisiana Field Well No. 1 were analyzedand the results are present below.

Procedure:

The core samples were prepared for testing in a fashion very similar tothat in steps 1-4 of Experiment 1 above. The test sequence began bydesaturating the core samples and measuring capillary pressures usingthe high speed centrifuge method. Capillary pressures were measured atfour pressure steps: 100, 200, 400, and 1000 psi. For capillarypressures above 1000, the present inventive method, outlined inExperiment 1, was used to determine capillary pressures.

High-pressure, mercury injection capillary pressure (MICP) testing wasthe last set to be completed due to the destructive nature of this test.Drainage capillary pressure measurements were completed using a117-pressure-step protocol—25 pressure steps in the low-pressure chamber(below atmospheric) and 92 pressure steps in the high-pressurechamber—corresponding to a measured capillary pressure range of 0 psigto 60000 psig. All mercury data were conformance corrected (low pressureend) for surface sample roughness, but were not blank corrected (highpressure end). Finally, the MICP were converted to an air-brine systemat laboratory conditions for comparison to the combined data from thecentrifuge method and vapor desorption method (according to the presentinvention).

Results:

FIG. 2 shows the combined capillary pressures from high speed centrifugedata and vapor desorption data (obtained according to the presentinventive method). FIG. 2 also shows the untransformed high-pressuremercury injection capillary pressures (MICP). As shown, the MICP curvesnot only exhibit higher capillary pressures, but also predict lowerwater saturations than the combined centrifuge and vapor desorptioncurves for a given capillary pressure.

FIG. 3 shows the same combined centrifuge and vapor desorption capillarypressure curves as shown in FIG. 2. With regard to the high-pressuremercury injection capillary pressures, the mercury injection method usesan air-mercury system in which mercury displaces air during theinjection process. Since this test is considered to be a drainageprocess (i.e., the wetting phase saturation is decreasing), then airmust be regarded as the wetting phase. In fact, neither air nor mercuryis a true rock wetting fluid under any condition. The present inventionshows a method to transform the MICP curves using the combinedcentrifuge and vapor desorption data (obtained according to the presentinventive method). The transform process of the present invention willnot only allow the salvaging of existing or legacy MICP data, but willalso provide more accurate MICP data to define the reservoir fluidsaturation distribution. The basis of the transform is a conversion ofthe air-mercury data to air-brine data (representative of the reservoir)using the following equation:

$P_{cAW} = {P_{cAM}\left( \frac{2\;\sigma_{AW}\cos\;\theta_{AW}}{2\;\sigma_{AM}\cos\;\theta_{AM}} \right)}$where:

P_(cAW)=pseudo air-water capillary pressure, psi

P_(cAM)=air-mercury capillary pressure, psi

σ_(AW)=air-water surface tension

σ_(AM)=air-mercury surface tension

cos θ_(AW)=air-water contact angle

cos θ_(AM)=air-mercury contact angle

The MICP curves of FIG. 2 were transformed by decreasing the σ_(AM) cosθ_(AM) term of the air-mercury system until the mercury curves closelymatched the vapor desorption curves. As shown in FIG. 3, aftertransformation, the MICP curves closely match the combined centrifugeand vapor desorption curves.

The present inventive method is advantageous over known methods ofdetermining high capillary pressures of a core sample because othermethods, centrifuge and porous plate, can not reach high capillarypressure, and high pressure mercury injection, while it can reach highcapillary pressure, is inaccurate especially at high capillarypressures. The vapor desorption method of the present invention providesan alternate method for extending capillary pressure measurements intothe high capillary pressure and ultra-low water saturation range. Thepresent invention provides for determining capillary pressures in excessof 10,000 psi air/brine yielding water saturations below 5%. The presentinventive method is well suited for reservoir systems characterized byultra-low water saturations and abnormally high capillary pressures suchas tight gas sands.

Also, the present invention is advantageous because it provides a methodfor producing core samples having high capillary pressure and low watersaturation which can then be tested concurrently for electricalproperties. There is no known method for producing core samples havingsuch high capillary pressures.

Further, the present invention is advantageous because it provides anaccurate method of transforming capillary pressure data determined fromhigh-pressure mercury injection which are generally known to beinaccurate especially at high capillary pressures.

All publications referred to herein are hereby incorporated by referencein their entireties.

Having described the invention above, various modifications of thetechniques, procedures, materials, and equipment will be apparent tothose skilled in the art. It is intended that all such variations withinthe scope and spirit of the invention be included within the scope ofthe appended claims.

1. A method for determining an electrical property of a core samplehaving a capillary pressure above about 4200 psi, comprising the steps:drying the core sample, weighing the core sample, saturating the coresample with a brine solution, weighing the core sample, calculating thebrine volume, placing the core sample in an electronically controlledhumidity chamber with temperature controls, setting a humidity level inthe humidity chamber at about 80% or lower, periodically weighing thecore sample to determine when equilibrium water saturation is achievedat the humidity level, calculating the capillary pressure at or aboveabout 4200 psi, and measuring an electrical property of the core sample.2. The method of claim 1, wherein the electrical property is formationfactor, resistivity index, cementation exponent or saturation exponent.